The present invention relates to a closed loop type fiber optic gyro and, more particularly, to a fiber optic gyro which permits a fault diagnosis to be made.
FIG. 1 shows a conventional closed loop type fiber optic gyro. A light source 12 is driven by a light source driver 11 to emit light. The light thus emitted from the light source 11 is provided via an optical coupler 13 to an optical splitter/coupler 15 which serves as interference means in an optical integrated circuit 14, and the light is incident therefrom, as right-handed and left-handed light, to both ends of an optical fiber coil 16 serving as an optical path which makes at least one loop. The right-handed light and the left-handed light emitted from the optical fiber coil 16 after propagation therethrough are combined by the optical splitter/coupler 15 and interfere with each other. The resulting interference light is provided via the optical coupler 13 to a photodetector 17 used as photoelectric conversion means for conversion into an electric signal corresponding to the intensity of the interference light. A bias phase modulator 18 is connected in series between the optical splitter/coupler 15 and one end of the optical fiber coil 16. A square-wave bias phase modulating signal from a bias phase modulator driver 19 is applied to the bias phase modulator 18 to drive it, by which the right-handed light and the left-handed light passing therethrough are both subjected to a +.pi./4 rad. phase shift and a -.pi./4 rad. phase shift alternately every propagation time .tau. of light through the optical fiber coil 16. The output of the photodetector 17 is synchronously detected by a synchronous detector 21 in synchronization with the phase shift.
When an angular rate having its center at the center of the optical fiber coil 16 is not being input thereinto, no phase difference exists between the right-handed light and the left-handed light both having propagated through the optical fiber coil 16. Upon input of the above-mentioned angular rate into the optical fiber coil 16, however, the following phase difference .phi..sub.R is introduced by the Sagnac effect between the right-handed light and the left-handed light, ##EQU1## where R is the radius of the optical fiber coil 16, L is the length of the optical fiber coil 16, .lambda. is the wavelength of the light from the light source 12, C is the velocity of light in a vacuum and .OMEGA. is the input angular rate. When the input angular rate .OMEGA. is zero, the output of the synchronous detector 21 is also zero, but when the input angular rate .OMEGA. develops, the synchronous detector 21 yields a DC output corresponding to the input angular rate .OMEGA.. The DC output is integrated by an integrator 22, whose output is applied to a ramp generator 23 to drive it. That is, the ramp generator 23 repeatedly generates a voltage which linearly varies to a predetermined value at a slope corresponding to the magnitude of the integrated output. By this ramp output the right-handed light and the left-handed light are phase modulated to cancel the phase difference .phi..sub.R in a feedback phase modulator 24 connected in series between the optical splitter/coupler 15 and one end of the optical fiber coil 16 and serving as phase difference generating means. As a result of this, the frequency of the output ramp voltage from the ramp generator 23 becomes proportional to the input angular rate .OMEGA. as follows: ##EQU2## where n is the refractive index of the optical fiber coil 16.
When the angular rate of forward rotation is provided to the optical fiber coil 16, the ramp generator 23 generates positive-going ramp waves and applies them to an output circuit 25, which, in turn, outputs a positive pulse for each ramp wave. When the angular rate of backward rotation is provided to the optical fiber coil 16, the ramp generator 23 applies negative-going ramp waves to the output circuit 25, which outputs a negative pulse for each ramp wave.
The conventional closed loop type fiber optic gyro described above has no self-diagnositc feature; therefore, once it is built in an apparatus, it is impossible to know whether the fiber optic gyro is normal, unless a check is made of the output of the apparatus after physically providing thereto an input angular rate or the fiber optic gyro produces an abnormal output owing to its failure.